Rotary dial telephones encode dialed digits by repetitively breaking the line current in a subscriber's loop, the number of breaks corresponding to the dialed digit. Dial pulse receivers and repeaters have from the early days of telephony utilized electromagnetic sensing of the dial pulses, by which a solenoid in a relay causes switch contacts to repetitively close, or in a stepping switch, contacts are caused to step along a row or column of switches.
While the repetitive line current breaking function theoretically provides square wave current pulses carried by the telephone line, in practice the form of the dial pulse current is usually far from perfect. Variations from the ideal square wave are usually caused by lines inductance, line leakage, leading and trailing edge ringing, cross talk, gliches, etc. While electromagnetic dial pulse receivers have been found to be somewhat tolerant of most of the imperfections in the waveform, more modern sensitive solid state dial pulse receivers have not had similar tolerance. Special circuitry has been incorporated into some solid state receivers for debouncing leading edge ringing waveforms, etc.
However it has been difficult to reliably detect dial pulses when there are gross distortions of the dial pulse current waveform. The present invention is directed to a reliable solid state circuit for dial pulse detection when the current pulses are grossly distorted.